Closed and open loop solar air collectors

ABSTRACT

A solar thermal collector is disclosed. The collector comprises a body defining a housing; a solar absorber located, within the housing; a transparent insulating medium in proximity to the solar absorber; and a heat exchanger integrated, with the housing.

FIELD

Embodiments of the invention relate to devices and methods to convert sunlight into heat.

BACKGROUND

Solar thermal collectors are devices which collect sunlight and convert it to heat. The resulting heat is then transferred to a fluid medium in the form of a gas or a liquid where it can be subsequently transported to a point of use. The transported heat can be exploited in a number of ways that are useful to human society and the many heat driven processes that support modern civilization.

SUMMARY

According to one aspect of the invention, there is provided a solar thermal collector incorporating a transparent insulating medium comprising, a nano-porous matrix: which allows sunlight to pass into the solar thermal collector while reducing losses through the medium which occur via conduction and radiation by virtue of the properties of the transparent insulating medium; the heat from which is subsequently transported to the exterior of the collector via an integrated heat exchanger.

According to another aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating, medium, comprising a nano-porous matrix: which allows sunlight to pass into the solar thermal system while reducing losses through the medium which occur via conduction and radiation by virtue of the way in which a heat transfer fluid interacts Within the transparent insulating medium; the heat from which is subsequently transported to the exterior of the collector via an integrated heat exchanger,

According to another aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium, comprising a nano-porous matrix: which allows sunlight to pass into the solar thermal system while reducing losses through the medium which occur via conduction and radiation by virtue of the heat transfer fluid being a gas or combination of gasses; the heat transfer fluid passing through the matrix; and the heat from which is subsequently transported to the exterior of the collector via an integrated heat exchanger.

According to another of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:

-   -   wherein, the heat transfer fluid is circulated within the         collector via thermally driven buoyancy forces; the heat         transfer fluid passing through the matrix;     -   and the heat from which is subsequently transported to the         exterior of the collector via an integrated heat exchanger.

According another aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:

-   -   wherein the heat transfer fluid is circulated within the         collector via a pump;     -   the heat transfer fluid passing through the matrix;     -   and the heat from which is subsequently transported to the         exterior of the collector via an integrated heat exchanger.

According another aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:

-   -   wherein the heat transfer fluid is circulated within the         collector via thermal transpiration;     -   the heat transfer fluid passing through the matrix;     -   and the heat from which is subsequently transported to the         exterior of the collector via an integrated heat exchanger.

According another aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:

-   -   wherein the heat transfer fluid is extracted from the ambient         environment;     -   the heat transfer fluid passing through the matrix;     -   and the heat from which is subsequently transported to the         exterior of the collector.

Other aspects of the invention will be apparent from the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are structural and schematic diagrams illustrating a solar thermal collector incorporating a transparent insulating medium, in accordance with on embodiment disclosed in a prior patent application.

FIG. 2 is a diagram illustrating a solar thermal collector incorporating a transparent insulating medium, and integrated heat exchanger, and associated flows of a heat transfer fluid circulating within the collector, in accordance with one embodiment of the invention.

FIG. 3 is a diagram illustrating a solar thermal collector incorporating a transparent insulating medium an integrated heat exchanger, and associated flows of a heat transfer fluid circulating within the collector, including a pump used to drive the flows in accordance with one embodiment of the invention.

FIG. 4 is a diagram which illustrates the phenomena of thermal transpiration.

FIG. 5 is a structural diagram which illustrates a solar thermal collector which extracts its heat transfer fluid from the ambient environment.

FIG. 6 is a structural diagram which illustrates a cylindrical solar thermal collector incorporating a transparent insulating medium and integrated heat exchanger, and associated flows of a heat transfer fluid circulating within the collector, in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will he apparent, however to one skilled in the art that the invention can be practiced without these specific details.

Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited b some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not others.

In one embodiment, a solar thermal collector is disclosed. The collector includes a light absorbing medium which converts incident sunlight into heat by raising the temperature of a heat transfer fluid, and fluidic pathways within the collector to allow for circulation of the heat transfer fluid to an integrated heat exchanger wherein the resulting heat can be transported to the exterior of the collector.

In general, the collector is nominally airtight, though some embodiments may allow for controlled exchange of air with the environment, and the interior under neutral pressure or below atmospheric pressure, in order to further minimize beat losses to the environment and enhance passive pumping mechanisms.

FIG. 1 shows a structural and schematic diagram of a solar thermal collector 100, in accordance with one embodiment of the invention disclosed in patent application co pending U.S. patent application Ser. No. 14/292,702 entitled “INSULATED SOLAR THERMAL SYSTEM” and herein incorporated by reference. More specifically FIG. 1a is a structural diagram of collector illustrating its basic components including a housing 102 which is hermetically sealed, a faceplate 104 through which sunlight may pass, an incoming cold heat transfer fluid 120 which passes through nano-porous medium 108 before coming into contact with solar absorber 110. Solar absorber 110 converts incident sunlight into heat which then raises the temperature of incoming heat transfer fluid 120 before it passes to the exterior of the collector as outgoing heated heat transfer fluid 116.

FIG. 1b reveals a collector with the same elements described with reference to FIG. 1a as well as additional components and details relating to interior fluid flow paths. In particular, the collector 100 is coupled to an external heat exchanger 118, via inter-connect piping send and return paths 116 and 120, respectively. Solar collector 100 is a device whose basic function is to convert sunlight into heat by raising, the temperature of a heat transfer fluid that comes into thermal contact with the solar absorber 110 as it passes through the collector 100.

Heat losses 112 from the collector are limited to some extent by the inclusion of the nano-porous medium 108, which has several properties which, make it useful in this role. These include transparency to most of the spectrum of light emitted by the sun and some degree of absorptivity or reflectivity for light wavelengths longer than those emitted by the sun. The medium 108 may be in the form of a planar monolithic structure with a planar surface area ranging from fractions of a meter to a meter or greater and a thickness of fractions of a centimeter to a centimeter or greater, Or a similarly sized and shaped matrix comprised of an aggregate of smaller particulates which may have sizes ranging from hundreds of microns to several millimeters or more in diameter. The particulates in the matrix may be of a smooth distribution in size within this range, or may have several distinct sizes. The geometries may be uniform and regular in nature or completely random.

The material comprising the nano-porous medium 108 may be in the form of an aerogel or aerogel like material made from a metallic oxide or other organic or inorganic materials whose fundamental structure consists of open or closed cellular pores whose diameter may range from tens of nanometers to tens of microns or more. The porous structure is such that it impedes the flow of a gas or gasses through the medium and therefore impedes the loss of heat from the solar thermal system. Other materials with similar properties including but not limited to masses of woven organic, or inorganic, fibers may be utilized and/or incorporated as well as long as some combination of the requisite transparency transparent to the solar spectrum and absorbing and/or reflective for wavelengths longer than the solar spectrum) porosity and thermally insulating characteristics (i.e. less than 0.04 W/m.K) can be achieved with the resulting medium. In this way heat losses 112 from the solar absorber 110 are mitigated thereby improving the efficiency of the solar thermal system.

During operation a cold heat transfer fluid is pumped via interconnect path 120 into collector 102 where it comes into contact with the solar absorber 110. The heat transfer fluid is nominally a gas or some combination of gasses, such as air, which have been selected due to their low-cost, their positive characteristics with respect to specific heat and thermal conductivity, and their benign nature from the standpoint of chemical reactivity and human health. In the embodiment shown, heat transfer fluid (introduced via the interconnect path 120 as described above) can come into contact with the solar absorber 110 via a combination of flow paths 122, 124, and 128, with the latter two paths causing flow directly through the nano-porous medium 108. The total flows are combined as they come into contact with solar absorber 110 and extracted via interconnect pipe 116 so that their heat can be used elsewhere. External heat exchanger 118, is located near or at the point of use for the heat and serves to transfer the heat from the heat transfer fluid, represented by flows 122, 124, and 126, to another heat transfer fluid or to the point of use.

Referring now to FIG. 2, reference numeral 200 generally indicates a variation on the solar collector 100. The collector 200 is similar in all respects to the collector 100 portrayed in FIG. 1. In the case of the collector 200 however, the external heat exchanger has been replaced by an integrated heat exchanger 132. Heat exchanger 132 is integrated in the sense that one portion of it resides within the interior of the collector housing and one portion of it resides outside the collector housing and it is a part of the physical construction of the collector's housing 102. Overall its function is to provide a mechanism to output heat from the interior of the collector to the exterior without compromising the insulating properties of the remainder of the collector's housing 102.

The heat exchanger 132 may be made from a variety of materials including, but not limited to metals, plastics, and ceramics. The overarching, requirements include that it can realize the requisite thermal conductivity to support heat transfer from the interior of the collector to the exterior, that it be environmentally rugged and be capable of supporting the hermeticity of the collector (if the collector is designed to be hermetically sealed), and that the surfaces of it which are to be exposed to internal and external heat transfer fluids be capable of being machined or fashioned in such a way as to achieve the requisite ratio of interior to exterior surface area ratio. For example, if the interior heat transfer fluid is to be a gas and the exterior heat transfer fluid is to be a liquid then the interior fluid will have a lower thermal conductivity than that of the exterior fluid. Crafting a heat exchanger with a high interior to exterior surface area ratio will facilitate the flow of heat from the interior to the exterior in this case by compensating for the differences in conductivity.

Referring back to FIG. 2. Heat is transferred to the heat exchanger 132 via internal heat transfer fluid flow 130 which as shown has been heated by coming into physical contact with solar absorber 110. Heat transfer fluid flow 130 passes into heat exchanger 132 where it loses heat and its temperature is lowered. The heat that is lost is transferred to incoming external heat transfer fluid 134, which emerges as the higher temperature outgoing external heat transfer fluid 136. The emerging cooled heat transfer fluid flow 138 returns to the interior of the collector housing and supplies the fluid source for flows 122, 124, and 128. The fluid is heated again, and the circulation of the fluid continues during the operation of the solar thermal collector.

Variations on internal heat transfer flow configurations are disclosed in the aforementioned patent application U.S. patent application Ser. No. 14/292,702, and are essentially defined by the ratios between the flow rates of fluid flows 122, 124, and 128 any one or two of which could have a value of zero as defined during the manufacture of the collector or during its operation under a control mechanism. All of aforementioned internal heat flow configurations apply to this embodiment with the only difference being that the internal heat transfer fluid is physically separated from the exterior of the collector.

Various methods can be used to pump the fluid within the collector. Referring again to FIG. 2 buoyancy forces may be used to achieve circulation via natural convection. Since the emerging fluid flow 130 will be heated and the fluid 138 emerging from the heat exchanger 132 will be cooled, the former will tend to rise and the latter will tend to sink. By proper configuration of the interior passages of the collector a naturally occurring circulation pattern may be set up to force a proper ratio of flows 122, 124 and 126. Properly configuring the internal passages of the collector refers generally to insuring that fluid flow 130 does not mix with fluid flows 138 which would result in minimizing the difference in temperature between the two flows and therefore the force of natural convection induced circulation. The force of convection is also determined by the orientation of the collector which is shown to be vertical (90 degrees) in FIG. 2. Under normal operation the collector will be fixed in a tilted position with the heat exchanger 132 being located higher than the opposite (or operatively lower) end of the collector housing. As the tilt is decreased (towards 0 degrees) the force of convection is reduced until it becomes virtually non-existent when the collector is horizontal i.e. parallel to the earth's surface. Thus, circulation using natural convection can only be realized when there is a collector tilt between 90 degrees (shown) and 0 degrees (normal to the sun).

Referring, now to FIG. 3 collector 300 is illustrated and is identical in every way to the collector illustrated in FIG. 2 except pump 140 is now included in the interior of the collector. Pump 140 is positioned so that it can serve to drive the fluid circulation within the collector and thus reduce the influence of collector tilt on the circulation of heat transfer fluid within the collector housing. For a fluid which is a gas, the pump would nominally take on the form of a fan, a small blower or other pump capable of forcing the flow of a gaseous fluid. The pump would nominally be driven by a source of electricity which could be from an external source or may be provided by a small photovoltaic cell which could be secured to the exterior of the collector or mounted separately so that sunlight striking it will cause the pump to operate during times when the collector is illuminated and converting sunlight into heat.

FIG. 4 illustrates the phenomena of thermal transpiration wherein gas may experience preferential pumping through a nano-porous medium if there is a temperature gradient placed across it. The temperature gradient is applied in the case of the solar thermal collector by virtue of the fact that the solar absorber (110 of FIG. 2.) is hot and therefore makes the side of nano-porous medium (108 of FIG. 2) which is closest to it much hotter than the side which is further away, resulting in a temperature gradient across the medium. Referring again to FIG. 4 gas flow through nano-porous medium 409 actually occurs in in a direction both towards and away from the hot side as illustrated by gas flow arrows 404 and 402 respectively. On average however more flow occurs in the direction towards the hot side. Thermal transpiration can thus be used to provide some or all of the pumping force required to circulate the internal heat transfer fluid within the housing of the collector. Factors which impact the significance of this effect include but are not limited to the thickness of the nano-porous medium, the size of the pores and the pore size distribution within the medium, the magnitude of the temperature gradient, and the pressure of the gas to be circulated. Some combination of these variables can be set during the manufacture of the collector or dynamically during its operation, by incorporation of the necessary components, to optimize the behavior of the collector in a static fashion, or dynamically during operation depending on the external conditions such as solar insolation, external temperature and wind induced heat loss, and heat output requirements.

In general, internal fluid circulation within the collectors illustrated in FIGS. 2 and 3 may be facilitated by any combination of buoyancy, thermal transpiration, and pump driven flows. The integration of the heat exchanger with the collector presented the challenge of insuring that any heat transport from the interior of the collector to the exterior occurs through the exchange of heat between the internal and external heat transfer fluids and not through heat loss paths through the collector housing via any mechanical supports, mounts, seals or the body of the heat exchange itself. This may require that the heat exchanger or the heat exchanger subassembly, comprising the heat exchanger and components designed to minimize loss through heat loss paths be manufactured from a combination of materials. These could include materials such as metals with high thermal conductivity to enhance heat exchange between the fluids, and materials such as plastics, foams, or even aerogel to with low thermal conductivity to reduce losses through the heat loss paths. Maintenance of the airtightness or hermiticily of the collector may be complicated by integrating the heat exchanger which, by virtue of being present, will introduce an opening in the collector's housing which must be sealed. Differences in thermally induced expansion rates between the heat exchanger and the collector's housing will introduce challenges that will require compensating, for any temperature induced difference in dimensions.

Another challenge to the design of this closed loop collector concerns changes in internal pressure as the collector undergoes heating. The consequence of a rise in temperature will mean an increase in the pressure. if the pressure in the non-operational state is near atmospheric it is doubtful that the collector an be easily designed to withstand the resulting increase thus some kind of expansion chamber will have to be incorporated into the collector. The expansion chamber is a component which is well known in the art of solar thermal systems as any heat transfer fluid in such systems will undergo expansion which results in an internal increase in pressure. In general, the expansion chamber is a metal container with a flexible membrane within that provides an airtight seal between openings on both sides of the membrane leading from the interior of the chamber to the exterior. If one of these opening is connected to a system which may undergo changes in pressure, then the membrane can expand within the chamber to accommodate the increased volume of the heat transfer fluid. if the collector is manufactured such that its internal pressure is below atmospheric then depending on the total increase in temperature during operation the interior pressure may not rise beyond atmospheric or not significantly past atmospheric. In such case an expansion chamber may not be necessary.

Referring now to FIG. 5 a structural diagram of a solar thermal collector 500 is shown which is functionally identical to the collector 300 shown in schematic diagram of FIG. 3 in the way in which sunlight is converted into heat and losses are reduced. However, in the collector 300 of FIG. 3 the heat transfer fluid is circulated entirely within the housing of the collector which is hermetically sealed preventing any direct physical interaction between the heat transfer fluid internally and any gasses or other environmental elements that are external to the collector. Collector 500 of FIG. 5 is supplied its heat transfer fluid 508 in the form of air which is extracted from the atmosphere via extraction module 502. Extraction module 502 performs a combination of functions which include at least filtering the incoming air of particulate matter in the form of dust, pollution, or other visible to microscopic airborne contaminants which could clog the pores of the nano-porous matrix 504. This can be done with a combination of air filters which are used within industry for this purpose and are well understood by those skilled in the art. The function could also include a valve which when closed can fluidically isolate the collector 500 from the environment during periods of operation. This could prevent the accumulation of water vapor within the collector which might be damaging in the long run. Output module 512, which provides a conduit path for the heated heat transfer fluid to be transported to a point of use, can perform a similar function by closing to isolate the collector from that side. The operation of valves in modules 502 and 510 can be driven electronically or mechanically via mechanisms that sense a non-operative state and thus automatically isolate the collector. Modules 502 and 510 may also include some mechanism for removing water vapor front within the collector either during operation or during the non-operative state. The latter is potentially more optimal in that less water vapor would be encountered during a non-operative state. Water vapor could be removed via desiccant or other means that is well known in the industry and it could be exposed to the interior environment of the collector after it has been fluidically isolated and begin the process of removing water vapor from the interior. The valves in modules 502 and 510 may operate in an on/off state or might be operated in an analog state in order to modulate the flow of the heat transfer fluid. The modules 502 and 510 may contain fans or pumps which provide the driving force to transport the air from the environment, through the collector and then to be output as heated heat transfer fluid 514. The modules may be coupled to a single collector or may be coupled to an array of collector. That is to say a single extraction module and output module can provide their respective functions to multiple collectors simultaneously. Such a system is well suited to providing heated process air to any number of industrial applications which can exploit it without the need for a heat exchanger.

That is to say that the heated heat transfer fluid 514, in the form of hot air, can be provided directly to an application such as drying food or textiles, heating the product, food Of textiles in this case, by direct contact with the heat transfer fluid 514.

Referring now to FIG. 6 a structural diagram of a solar thermal collector 600 is shown which is functionally identical to the collector 300 shown in schematic diagram of FIG. 3 in the way in which sunlight is converted into heat and losses are reduced. The only difference in this case is that solar thermal collector 600 is cylindrical in shape vs, planar rectangular. Glass tube housing 602 contains cylindrical annulus 604, which surrounds nano-porous medium 606 which surrounds solar absorber 608 which is cylindrical and hollow. All of these components are made from the similar if not identical materials and have similar if not identical thermal and optical properties to their counter parts in FIG. 6. Glass tube housing 602 is mechanically integrated with heat exchanger 611 in such a way that the interior is hermitically sealed from the environment. During operation sunlight is incident on solar absorber 608 which converts sunlight into heat which is subsequently transferred to heat transfer fluid 610 (indicated by the associated arrows which also serve to illustrate the three primary flow paths) which flows through nano-porous medium 606 before coming into physical contact with solar absorber 608 rows. The resulting heated heat transfer fluid indicated by arrow 612, flows into heat exchanger 611 where it is cooled by losing its heat and subsequently re-circulated back into annulus 604 where it can then flow again, through nano-porous medium 606 in a continuous cycle. As in solar thermal collector 300 of FIG. 3, the flow of the heat transfer fluid may be driven by some combination of pump, buoyancy and thermal transpiration forces. As in solar thermal collector 300 of FIG. 3 an external cooled heat transfer fluid, indicated by arrow 616, enters the heat exchanger 611 where it is heated and emerges at an elevated temperature as indicated by arrow 618. The nano-porous medium may be composed of a particulate medium or a monolithic one which has be manufactured to achieve a geometry that fits within the collector's cylindrical form.

Overall, integrating the heat exchanger has the potential to make it easier to interface the collector to existing thermal piping and interconnect systems without the requirement of changing the heat transfer fluid within the piping system. Systems utilizing the collector design illustrated in FIG. 1 will of necessity have to utilize the same heat transfer fluid within the piping system, illustrated by conduits 116 and 120 of FIG. 1 a. Most piping systems for solar thermal applications utilize thermal oil or a water/glycol solution as the heat transfer fluid, If the piping system represents a significant asset such that the economics of altering it to accommodate a different heat transfer fluid are untenable then utilizing the collectors as defined in FIGS. 2, 3 and 6 could prove beneficial. 

1. A solar thermal collector, comprising: a body defining a housing; a solar absorber located within the housing; a transparent insulating medium in proximity to the solar absorber; and a heat exchanger integrated with the housing.
 2. The collector of claim wherein at least a portion of the heat exchanger resides within the housing.
 3. The collector of claim 1, wherein the transparent insulating medium comprises a nano-porous matrix
 4. The collector of claim 3, wherein the solar absorber is embedded within the nano-porous matrix.
 5. The collector of claim 4, wherein the nano-porous matrix comprises aerogel,
 6. The collector of claim 1, further comprising an inlet and outlet defined between the heat exchanger and the body defining the housing; wherein the inlet serves to introduce a cold heat transfer fluid into the housing, to undergo heating that coming into contact with the solar absorber, and the outlet serves to allow the heated heat transfer fluid to pass from the collective body two the heat exchanger.
 7. The collector of claim 6, wherein the heat transfer fluid is circulated within the housing thermally-driven buoyancy forces.
 8. The collector of claim 6, further comprising a pump located within the housing configured to facilitate circulation of the heat transfer fluid.
 9. The collector of claim 1, wherein the body comprises a planar configuration including upper and lower faceplates.
 10. The collector of claim 1, wherein the body has a cylindrical configuration.
 11. The collector of claim 4, wherein the heat transfer fluid comprises air.
 12. The collector of claim 1, wherein the heat exchanger is elevated relative to the solar absorber.
 13. The collector of claim 1, which forms a closed loop system.
 14. The collector of claim 1, which forms an open loop system. 